Organ and tissue transplantation is a unique immunologic situation because both donor and recipient antigen presenting cells (APCs) are present at the time of reperfusion. The donor APCs are immune cells that become trapped within an allograft at the time of organ retrieval as part of normal immune surveillance. The donor APCs are referred to as passenger leukocytes. The passenger leukocytes migrate out of the allograft within hours of re-implantation and traffic to the secondary lymphoid tissues of the patient where they serve as MHC class II positive APCs stimulating direct activation of recipient naive CD4+ T cells (known as the direct pathway of allorecognition). The ability of passenger leukocytes to activate recipient T cells can be attributed in part to the expression of costimulatory molecules on the cell surface of the passenger leukocytes that interact with receptor ligands on the surface of T cells. Key co-stimulatory molecules found on the surface of passenger leukocytes include the CD80 and CD86 proteins which provide a co-stimulatory signal to the CD28 T cell receptor; and the CD40 protein which provides a co-stimulatory signal to the CD154 T cell receptor. Once activated, the patient's T cells clonally expand and traffic to the donor transplant site leading to acute transplant rejection. Direct antigen presentation by the passenger leukocytes is most potent during the first week post-transplantation. Direct T cell priming is time limited because the passenger leukocytes are eventually destroyed.
Much work has focused on preventing T cell activation following transplantation by attempting costimulatory blockade within the recipient patient. Current therapies focus on using immunosuppressive regimens that impact the effector arm of the recipient's immune system (eg. by preventing activation and proliferation of naïve CD4+ T cells following interaction with the donor passenger leukocytes, and blocking their infiltration into the allograft). However, when costimulatory blockade has been attempted in vivo, toxicity is a complicating issue. For example, whilst monoclonal antibodies targeting CD28, CD40 and CD40L have great potential to ameliorate allograft rejection, these therapies have been associated with severe complications when administered systemically. The side-effects have included cytokine storm, disseminated intravascular coagulation and thrombotic complications that have led to significant morbidity and mortality. An additional complication of systemic administration of monoclonal antibodies is the suppression of viral immune responses that can result in severe systemic manifestations. An approach postulated to enhance the efficacy of costimulatory blockade entails combining treatments to address several costimulatory pathways simultaneously. Such a combined approach could provide better efficacy to address the redundancy existing in T cell costimulatory pathways. However, given the potential risks associated with in vivo administration of a single therapeutic to blockade a single pathway, blockade of multiple pathways may present significant increased risks of complications.
Current therapies for preventing transplant rejection therefore focus entirely on targeting the recipient's immune system, rather than attempting to prevent antigen presentation by targeting the passenger leukocytes themselves. Indeed, current immunosuppressive drugs do not appreciably alter the migration of the donor passenger leukocytes out of an allograft and into the secondary lymphatics of the recipient, nor do they ameliorate the immune response initiated by the passenger leukocytes in the recipient's secondary lymphatics.
If direct antigen presentation mediated by passenger leukocytes can be prevented during the early posttransplant period, allosensitization leading to acute rejection may be prevented. Furthermore, if a population of passenger leukocytes can survive within the patient without inducing antigen presentation, a state of immune tolerance may be induced. Indeed, survival of even rare populations of passenger leukocytes could result in a tolerate state. The dissemination of the donor passenger leukocytes entails migration to the secondary lymphatics for a period of approximately 14 days after which they migrate into other tissues in the recipient. The survival of even a rare population of the donor passenger leukocytes has been postulated to be the basis of micro-chimerism that is associated with a tolerant state.
So far, only modest efforts have been made to focus on treating the allograft itself, prior to transplantation. Attempts to date to remove passenger leukocytes from human organs have not been successful.
There is therefore a need in the art for new ways of targeting the passenger leukocyte population in donor tissues and organs to prevent allorecognition and transplant rejection.